Iridium coatings

Platinum—Iridium. There are two distinct forms of 70/30 wt % platinum—iridium coatings. The first, prepared as prescribed in British patents (3—5), consists of platinum and iridium metal. X-ray diffraction shows shifted Pt peaks and no oxide species. The iridium [7439-88-5] is thus present in its metallic form, either as a separate phase or as a platinum—iridium intermetallic. The surface morphology of a platinum—iridium metal coating shown in Figure 2 is cracked, but not in the regular networked pattern typical of the DSA oxide materials. 

The second form consists of Pt metal but the iridium is present as iridium dioxide. Iridium metal may or may not be present, depending on the baking temperature (14). Titanium dioxide is present in amounts of only a few weight percent. The analysis of these coatings suggests that the platinum metal acts as a binder for the iridium oxide, which in turn acts as the electrocatalyst for chlorine discharge (14). In the case of thermally deposited platinum—iridium metal coatings, these may actually form an intermetallic. Both the electrocatalytic properties and wear rates are expected to differ for these two forms of platinum—iridium-coated anodes. 

P. S. S. Hayfteld and W. R. Jacob, "Platinum—Iridium-Coated Titanium Anodes ia Brine Electrolysis," paper presented 2i. A.dvances in Chlor-A.lkali Technology, London, 1979. 

Harding, J. T., Kazarof, J. M., and Appel, M. A., Iridium-coated Rhenium Thrusters by CVD, NASA Technical Memo. 101309, NASA Lewis Res. Cent., Cleveland, OH (1988)... 

Iridium Coating for Spacecraft Rocket Nozzles. The coating of rocket nozzles with iridium is a good example of the ability of CVD to provide a complete composite material, in this case a structural refractory shell substrate coated with a corrosion- and oxidation-resistant component. The device is a thruster rocket nozzle for a satellite. The rocket uses a liquid propellant which is a mixture of nitrogen tetroxide and monomethyl hydrazine. 

Pozebon, D., Dressier, V. L., and Curtius, A. J. (1998). Determination of arsenic, selenium and lead by electrothermal vaporization inductively coupled plasma mass spectrometry using iridium-coated graphite tubes.J. Ar/a/. At. Spectrom. 13(1), 7. 

Under severe conditions and at high temperatures, noble metal films may fail by oxidation of the substrate base metal through pores in the film. Improved life may be achieved by first imposing a harder noble metal film, eg, rhodium or platinum—iridium, on the substrate metal. For maximum adhesion, the metal of the intermediate film should ahoy both with the substrate metal and the soft noble-metal lubricating film. This sometimes requires more than one intermediate layer. For example, silver does not ahoy to steel and tends to lack adhesion. A flash of hard nickel bonds weh to the steel but the nickel tends to oxidize and should be coated with rhodium before applying shver of 1—5 p.m thickness. This triplex film then provides better adhesion and gready increased corrosion protection. 

Commercial metal anodes for the chlorine industry came about after the late 1960s when a series of worldwide patents were awarded (6—8). These were based not on the use of the platinum-group metals (qv) themselves, but on coatings comprised of platinum-group metal oxides or a mixture of these oxides with valve metal oxides, such as titanium oxide (see Platinum-GROUP metals, compounds Titanium compounds). In the case of chlor-alkaH production, the platinum-group metal oxides that proved most appropriate for use as coatings on anodes were those of mthenium and iridium. 

Iridium Oxide. Iridium dioxide [12030 9-8] coatings, typically used in combination with valve metal oxides, are quite similar in stmcture to those of mthenium dioxide coatings. X-ray diffraction shows the mtile crystal stmcture of the iridium dioxide scanning electron micrographs show the micro-cracked surface typical of these thermally prepared oxide coatings. 

Fig. 2. Scanning electron microscope photograph of platinum—iridium metal coating on titanium.

A dimensionally stable anode consisting of an electrically conducting ceramic substrate coated with a noble metal oxide has been developed (55). Iridium oxide, for example, resists anode wear experienced ia the Downs and similar electrolytic cells (see Metal anodes). 

Iridium on valve metals is suitable if the consumption rate of platinum is too high at elevated temperatures or critical composition of the medium. Mostly platinum-iridium alloys are used with about 30% Ir, because coating valve metals with pure iridium is somewhat complicated. For the same reason, other noble metals such as rhodium cannot be used [21]. At present there is little price difference between platinum and iridium. 

In 1996, consumption in the western world was 14.2 tonnes of rhodium and 3.8 tonnes of iridium. Unquestionably the main uses of rhodium (over 90%) are now catalytic, e.g. for the control of exhaust emissions in the car (automobile) industry and, in the form of phosphine complexes, in hydrogenation and hydroformylation reactions where it is frequently more efficient than the more commonly used cobalt catalysts. Iridium is used in the coating of anodes in chloralkali plant and as a catalyst in the production of acetic acid. It also finds small-scale applications in specialist hard alloys. 

Rhodium and iridium have a resistance to anodic corrosion comparable with that of platinum, and are more resistant to the influence of alternating currents. A platinum-iridium alloy, in the form of a coating on titanium, is preferred to pure platinum for the production of chlorine from brine , due to its improved corrosion resistance and lower overvoltage. 

The composition of the mixed metal oxide may well vary over wide limits depending on the environment in which the anode will operate, with the precious metal composition of the mixed metal oxide coating adjusted to favour either oxygen or chlorine evolution by varying the relative proportions of iridium and ruthenium. For chlorine production RuOj-rich coatings are preferred, whilst for oxygen evolution IrOj-rich coatings are utilised. ... 

The coating composition is iridium-rich to favour oxygen rather than chlorine evolution, and to assist in reducing the formation of acidic conditions at the anode-concrete interface. 

The most widely used methods for the application of coatings of gold, silver and the platinum group metals (platinum, palladium, rhodium, iridium, ruthenium, osmium) to base metals are mechanical cladding and electroplating. 

Ruthenium, iridium and osmium The use of a fused cyanide electrolyte is the most effective means for the production of sound relatively thick coatings of ruthenium and iridium, but this type of process is unattractive and inconvenient for general purposes and does not therefore appear to have developed yet to a significant extent for industrial application. This is unfortunate, since these metals are the most refractory of the platinum group and in principle their properties might best be utilised in the form of coatings. However, several interesting improvements have been made in the development of aqueous electrolytes. 

Iridium has been deposited from chloride-sulphamate and from bromide electrolytes , but coating characteristics have not been fully evaluated. The bromide electrolytes were further developed by Tyrrell for the deposition of a range of binary and some ternary alloys of the platinum metals, but, other than the platinum-iridium system, no commercial exploitation of these processes has yet been made. 

Platinum Platinum-coated titanium is the most important anode material for impressed-current cathodic protection in seawater. In electrolysis cells, platinum is attacked if the current waveform varies, if oxygen and chlorine are evolved simultaneously, or if some organic substances are present Nevertheless, platinised titanium is employed in tinplate production in Japan s. Although ruthenium dioxide is the most usual coating for dimensionally stable anodes, platinum/iridium, also deposited by thermal decomposition of a metallo-organic paint, is used in sodium chlorate manufacture. Platinum/ruthenium, applied by an immersion process, is recommended for the cathodes of membrane electrolysis cells. ... 

The nozzle of original design was fabricated from a niobium alloy coated with niobium silicide and could not operate above 1320°C. This was replaced by a thin shell of rhenium protected on the inside by a thin layer of iridium. The iridium was deposited first on a disposable mandrel, from iridium acetylacetonate (pentadionate) (see Ch. 6). The rhenium was then deposited over the iridium by hydrogen reduction of the chloride. The mandrel was then chemically removed. Iridium has a high melting point (2410°C) and provides good corrosion protection for the rhenium. The nozzle was tested at 2000°C and survived 400 cycles in a high oxidizer to fuel ratio with no measurable corrosion.O l... 

---